Plasma generation is useful in a variety of microelectronic fabrication processes, including etching, resist stripping, passivation, deposition, and the like. There are a variety of plasma techniques known for processing microelectronic devices. Most known applications for using plasma can be significantly enhanced if the density of the plasma can be increased and maintained at low pressures, particularly with increasing miniaturization of features of microelectronic devices.
For example, plasma etching is a technique known in the art for patterning a substrate surface to form microelectronic devices and their interconnections. Ideally, substrates are etched primarily in a direction orthogonal to the surface thereof, i.e., in a direction perpendicular to the surface of the substrate. As a material is etched, walls are formed in the material, generally referred to as sidewalls. Ideally etching continues in a substantially vertical direction to the substrate and not laterally into the thus created sidewalls. Such vertical etching with minimal or no lateral etching into the sidewalls is referred to as anisotropic etching.
Plasmas pattern substrates by using directed ions to achieve anisotropic etching. To provide ions with sufficient directionality requires plasma operation at low pressures, typically about 1-20 mTorr. This prevents scattering of the ions by collisions with the gas molecules. Further, it is advantageous to operate plasma processes at high rates for commercial viability. This necessitates the use of high density plasmas.
Currently, several techniques can be used to generate high density plasmas at low pressures. For example, plasma sources such as electron cyclotron (ECR), helicon or MORI, and inductively coupled (RFI/TCP/ICP) sources are becoming increasingly important for plasma processing applications due to their ability to operate at low pressure and high plasma density. ECR processes couple a microwave energy source with a magnetic field to create electron cyclotron resonance in the electrons of a gas to generate a plasma of the gas. This, however, can require high outputs of energy to maintain the plasma. That is, ECR sources typically operate at 2.45 GHz and require an 875 Gauss magnetic field with its attendant costs for coils, power supplies, cooling, and power.
Other techniques generate a plasma by generating helicon or whistler waves in a gas. The helicon waves can be formed by coupling a magnetic field with RF energy, using complicated, large volume antenna structures. This in turn requires complex and large scale reactor design. For example, helicon sources typically use antenna structures external to a cylindrical plasma column to set up either an m=0 or m=.+-.1 mode with a well defined parallel wavelength. This geometry usually necessitates a separate source and downstream processing chamber. Helicon sources typically operate at magnetic fields of 100-400 Gauss because the plasma density, magnetic field, and wavenumber are linked by the helicon dispersion relation.
Still other techniques use inductively coupled plasma sources, which include a flat coil as the coupling element to generate the plasma. Compact reactors can be constructed from inductively coupled plasma generators. However, the plasma is generated near the window and typically has a planar, or "pancake" shape. This can result in small degree of selectivity in etching operations, because of the small chemical reaction surface area of the chamber. RFI/TCP sources with flat spiral coils couple power primarily inductively to the plasma with the RF power deposited primarily within half a skin depth (approximately 0.5-4 cm) of the window, while ECR and helicon sources are wave supported and deposit their power in the plasma bulk.
U.S. Pat. No. 4,810,935 to Boswell discloses a method and apparatus for producing large volume, uniform, high density magnetoplasmas for treating (e.g., etching) microelectronic substrates. The high density plasma is generated using whistler or helicon waves. The plasma is created in a cylinder. A coil is wrapped about the cylinder to create a magnetic field in the cylinder. An antenna is located alongside the cylinder. RF energy, the source of power used to establish the plasma, is coupled to the cylinder by the antenna.
U.S. Pat. No. 4,990,229 to Campbell et al. discloses another apparatus for forming a high density plasma for deposition and etching applications. Again, whistler or helicon waves are used to generate the plasma. The apparatus includes a cylindrical plasma generator chamber. A pair of coils produce an axial magnetic field within chamber. An antenna is mounted on the chamber, to launch RF waves at low frequency along the magnetic field. The plasma is transported by the magnetic field to a separate processing chamber.
U.S. Pat. No. 5,225,740 to Ohkawa discloses a method and apparatus for producing high density plasma using helicon or whistler mode excitation. The high density plasma is produced in a long cylindrical cavity imbedded in a high magnetic field, generated by a coil wound around the plasma chamber. In one embodiment, electromagnetic radiation is coupled axially into the cylindrical cavity using an adjustable resonant cavity to excite a whistler wave in the cylindrical cavity and hence in the plasma. In another embodiment, electromagnetic radiation is coupled radially into the cylindrical cavity using a slow wave structure to excite the whistler wave in the plasma. In both embodiments, the plasma is generated without using electrodes.
U.S. Pat. No. 5,146,137 to Gesche et al. discloses a device for the generation of plasma by means of circularly polarized high frequency waves, such as whistler and helicon waves. The apparatus includes a plasma chamber having an upper cylinder and a lower cylinder, an antenna, and a coil, both of which are placed about the upper cylinder. The coil creates a magnetic field. The antenna generates waves which are coupled through the magnetic field in the plasma in the helicon state. Four electrodes generate an electromagnetic field. First and second voltages, which are phase-sifted by 90.degree., are applied to opposing pairs of electrodes to develop the helicon wave.
U.S. Pat. No. 4,948,458 to Ogle discloses an inductively coupled plasma apparatus. The apparatus includes a generally air tight interior chamber within which the plasma is generated. To induce the desired plasma, an electrically conductive coil is disposed adjacent to the exterior of the enclosure. The coil is substantially planar, including a single conductive element formed into a planar spiral or a series of concentric rings. By inducing a radio frequency current within the coil, a magnetic field is produced which will induce a generally circular flow of electrons within a planar region parallel to the plane of the coil.
Despite these and other plasma processing techniques and apparatus, there exists a need for a plasma process and apparatus which can be used to produce and maintain high density plasma under low pressure conditions. Further, it would be desirable to provide such plasmas without requiring high outputs of energy to maintain the plasma or requiring complicated, large volume antenna structures, and thus complex and large scale reactor design. In addition, it would be desirable to provide an apparatus which includes a single chamber in which the plasma can be generated and the microelectronic substrate processed, and which includes a large chamber wall surface area, and thus a large chemical reaction surface area, to thereby increase the degree of selectivity in etching operations at low pressures.